Field of the Invention
The present invention relates to a method for producing amorphous carbon particles, amorphous carbon particles, a negative electrode material for a lithium ion secondary battery, and a lithium ion secondary battery.
Discussion of the Background
Conventionally, mainly from the viewpoint of price and weight reduction, nickel metal hydride batteries have been used as rechargeable batteries for hybrid cars. In order to further reduce weight, the use of lithium ion secondary batteries which have a high voltage and a high energy density has been anticipated.
Regarding batteries for cars driven by batteries only, such as electric cars, in order to ensure a sufficient distance per charge, the use of materials having a high energy density, and graphite-based materials in negative electrodes, has been widely studied.
On the other hand, in a system in which the volume to be loaded in a car is small and it is necessary to regenerate energy during deceleration by braking, such as in a battery for a hybrid car, there has been a demand for batteries having high charge/discharge specific output density, and use of amorphous carbon particles, a representative example of which is hard carbon, has been studied. Background art relating to the amorphous carbon particles is described in, for example, Japanese Unexamined Patent Application Publication No. 3-252053, Japanese Unexamined Patent Application Publication No. 6-89721, Japanese Unexamined Patent Application Publication No. 8-115723, and Japanese Unexamined Patent Application Publication No. 9-153359.
Amorphous carbon particles have a low true specific gravity compared with graphite particles (i.e., when measured in accordance with a method for measuring true specific gravity using butanol, the true specific gravity of a graphite-based material is about 2.23, while the true specific gravity of common hard carbon is low at 1.5 to 1.6 although depending on the material), and amorphous carbon particles are hard particles. Therefore, it is difficult to improve the electrode density, and the electrode press formability may degrade in some cases.
Furthermore, amorphous carbon particles tend to have low conductivity compared with graphite particles. Improvement in conductivity is believed to be an effective measure to improve charge/discharge, input/output characteristics per second in which electron conductivity mainly dominates.
Furthermore, in the process of fabricating an electrode using amorphous carbon particles, it is also conceivable to add afterwards graphite particles having good conductivity, as a conductive auxiliary material. However, for example, in the case where an electrolyte solution containing propylene carbonate which has excellent low-temperature characteristics and which does not solidify even in cold climates is used, reaction (decomposition reaction) occurs between graphite particles and the electrolyte solution, and charging is not performed, adversely affecting the battery performance.